Scientists may have discovered the key to why some twins are born identical, a milestone that could help develop treatments for congenital disorders that affect identical twins more often. An international team of scientists at Vrije Universiteit in Amsterdam found a DNA signature shared by identical twins, the Guardian reports. They focused on epigenetic factors, which are external, on top of genetics, and can turn genes off or on without affecting the gene sequence. The switching affects the way cells read the genes, per Live Science. The reason a fertilized egg splits into two embryos with precisely the same genes has never been known.
The scientists looked at the genomes of more than 6,000 people around the world—identical twins, fraternal twins, and family members who aren't twins. Identical twins were found to have the same signature DNA tags at 834 points among 450,000 sites across their DNA, per Mashable. The researchers, who published their work in Nature Communications, said the method was so reliable that they could tell from the shared marks whether a person is an identical twin with 80% accuracy. That would tell people whether they'd been separated from a twin at birth or if their twin had been lost in the womb.
The finding isn't the final word on twins. One American epigeneticist not involved in the study thinks the epigenetic markers probably result from the twin-splitting process, per Science News. Another theory is that they're a factor in causing it, per Popular Science; more research could help with that answer. The study is still a step toward understanding twins, experts said. A developmental psychologist at California State University said the findings are "very, very important" in understanding why identical twins more often have conditions such as spina bifida. In some cases, people who have the afflictions might not know they're a twin. (More identical twins stories.)
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